Diatoms and their influence on the biologically mediated uptake of atmospheric CO₂ in the Arabian Sea upwelling system

Abstract. Sediment trap experiments have been carried out in order to study processes controlling shifts from diatom to non-diatom dominated systems in the western Arabian Sea. One of our major problems was to link sediment trap records to surface ocean processes. Satellite-derived observations on upper ocean parameters were helpful to reduce this problem in the past and gain a new quality by combining it with results obtained during the Joint Global Ocean Flux Study (JGOFS) in the Arabian Sea. The new results imply that intense grazing can decline or impede the development of large diatom blooms in a silicon-enriched near shore upwelling system off Oman. In the open western Arabian Sea diatom blooms recover within the offshore advecting upwelled water and lead to peak organic fluxes into the deep sea but only during the later phase of the upwelling season. During onset of the upwelling season grazing favoured by eolian iron inputs causing the formation of thinner diatom shells seems to prevent the development of a large diatom bloom within the silicon-enriched offshore advecting upwelled water. An increased relevance of diatoms and diatom-grazing copepods in the planktonic community as well as oligotrophic conditions seem to raise the ratio between organic carbon formation and calcium carbonate carbon precipitation (rain ratio) in the surface water. The decomposition of organic matter in the water column reduces the rain ratio within in the sinking matter especially in the oligotrophic region dominated by cyanobacteria and reduces also the variation of the carbon to nutrient uptake ratios seen in the surface water.